A hydraulic shock absorber, used in particular in a motor vehicle, makes it possible to achieve an appreciable increase in the grip of the tires of the wheels of the motor vehicle with the ground while at the same time damping the movements of the suspended mass of the vehicle. Such a device generally comprises a cylinder filled with hydraulic fluid, inside which there may move a main piston actuated by a rod. The piston defines within the cylinder a first chamber and a second chamber containing the rod. When the piston moves inside the cylinder, the immersed volume of the rod varies, and that leads to a variation in the volume available for the oil inside the cylinder. A reservoir of hydraulic fluid is therefore connected to the cylinder in order to compensate for these variations.
During a compression movement in which the rod enters the cylinder, the hydraulic fluid is displaced by the main piston from the first chamber toward the second chamber. In order to generate a hydraulic force opposing the movement of the suspension, the displaced fluid is passed through restrictions which may be of various types. It is possible to use simple calibrated orifices, check valves made up of foil or light metal components, or even special-purpose valves.
In a conventional shock absorber, the damping force is dependent on the rate of compression or relaxation. Such a response may give rise to problems during a compression movement of the shock absorber. A sudden acceleration experienced by the wheel, for example when passing over small raised obstacles on the roadway, may generate a significant variation in the damping force, it being possible for the resulting jolt to be felt unpleasantly inside the vehicle. In order to avoid such jolts, the damping is generally reduced during a compression. There is then the risk that the damping will not be sufficient to provide suitable control over the movements of the vehicle bodyshell. It is possible to consider increasing the damping during a relaxation in order to overcome this difficulty, but this then results in a risk of a loss of grip when the roadway is rutted, as the wheel of the vehicle will no longer be able to follow the profile of the roadway.
A shock absorber device described in French patent application FR 2 838 172 comprises a valve having a valve shutter. Filtering means are mounted in parallel with the valve in order to generate a control pressure acting on the valve shutter. In such a shock absorber, the variation in pressure between the first and second main chambers of the cylinder of the shock absorber are used to generate, via the filtering means, a control pressure that acts upon the valve shutter. The percussion phenomena encountered in conventional shock absorbers are thus filtered.
Such a shock absorber performs extremely well. However, its structure is complex which means that mass production thereof is difficult to achieve.
The subject of the present invention is a shock absorber capable of overcoming these problems.